1. Field of the Invention
The present invention relates to a production method of a multilayer ceramic substrate having a cavity, particularly to an improvement in a method for producing a multilayer ceramic substrate having a cavity utilizing a non-shrinkage firing process.
2. Description of the Prior Art
In the fields of electronics or other such devices, substrates for mounting electronic devices thereon have been being widely used. In recent years, however, multilayer ceramic substrates have been proposed and put to practical use as substrates satisfying demands for reduction in size and weight and for multifunctionality and having high reliability. A multilayer ceramic substrate is constituted by a plurality of ceramic layers laminated, and integral incorporation of a wiring conductor or an electronic device into each ceramic layer enables a substrate to be highly dense.
With respect to the multilayer ceramic substrates, with an aim of facilitating miniaturization and reduction in height of electronics, multilayer ceramic substrates having a cavity (concave part) formed therein for accommodating an electronic device have also been put to practical use. Since the multilayer ceramic substrate provided with a cavity can be mounted as having an electronic device accommodated in the cavity, the aforementioned demands can satisfactorily be fulfilled, thereby making it possible to realize reduction in size and height of the multilayer ceramic substrate per se.
Incidentally, the multilayer ceramic substrate can be obtained through the steps of laminating a plurality of green sheets to form a multilayer body and firing the multilayer body. The green sheet always shrinks as accompanied by sintering in the firing step. This is a serious cause in decreasing the dimensional accuracy of the multilayer ceramic substrate. To be concrete, a shrinkage variation arises as accompanied by the shrinkage and, in the multilayer ceramic substrate being obtained finally, the dimensional accuracy falls around 0.5%.
Under these circumstances, a so-called non-shrinkage firing process capable of suppressing the shrinkage in the in-plane direction and shrinking only in the thickness direction of the green sheets in the firing step of the multilayer ceramic substrate was proposed (JP-A HEI 10-75060, for example). As described in the prior art, when a multilayer body of green sheets having attached thereto a sheet not shrinkable even at the temperature in the aforementioned firing step is fired, the shrinkage in the in-plane direction is suppressed and only the shrinkage in the thickness direction is produced. According to this process, the dimensional accuracy in the in-plane direction of a multilayer ceramic substrate can be improved to fall around 0.05%.
When fabricating a multilayer ceramic substrate having the cavity mentioned above, even an application of the non-shrinkage firing process poses a problem of not always obtaining satisfactory dimensional accuracy or satisfactory flatness. This is because according to the ordinary non-shrinkage firing process the binding force of shrinkage suppression is not exerted onto the bottom of the cavity. When the binding force of shrinkage suppression is not exerted onto the bottom of the cavity, flatness of the bottom required for mounting an electronic device thereon cannot be secured to the effect that there is a possibility of failing to mount the electronic device on the bottom.
In view of the above, an attempt to also attach a shrinkage-suppressing sheet onto the bottom of the cavity was made to eliminate the disadvantage (JP-A 2003-318309, for example). The method of the prior art comprises the steps of forming on a carrier film a shrinkage-suppressing sheet containing an inorganic material for shrinkage suppression, inserting in the shrinkage-suppressing sheet a cut of the shape corresponding to the contour of the bottom of the cavity, removing the portion of the sheet outside the cut, transferring the shrinkage-suppressing sheet retained on the carrier sheet onto a ceramic green sheet for a substrate (substrate ceramic green sheet) that will constitute the bottom of the cavity in the step of laminating ceramic green sheets for fabricating a crude multilayer body for the substrate and performing the firing step, with the shrinkage-suppressing sheet disposed on the bottom of the cavity. With this, it is made possible to heighten the dimensional accuracy, difficult to form undesirable distortion within the cavity and possible to attain high density of wiring with high reliability.
However, only disposition of the shrinkage-suppressing sheet on the ceramic green sheet for the substrate that will constitute the bottom of the cavity is difficult to completely eliminate the problem of the deformation etc. of the cavity. Particularly, the method for producing a multilayer ceramic substrate requires a step of pressing a multilayer body having a plurality of green sheets laminated and, in the pressing step of the method for producing a multilayer ceramic substrate having a cavity, there is a fair possibility of the cavity opening being collapsed and deformed in the pressing step. Also in the firing step, a phenomenon of rendering the periphery of the cavity opening to bulge will occur, thus raising a possibility of the cavity opening being deformed.
As another method for fabricating a multilayer ceramic substrate having a cavity, also conceivable is a method comprising the steps of laminating a plurality of green sheets to form a multilayer body, firing the multilayer body and subjecting the multilayer body to a boring process. However, since the sintered multilayer body is hard and fragile, a process with high accuracy is difficult to perform and expensive equipment is required to use, leading to a high production cost.
The present invention has been proposed in view of the conventional state of affairs, and the object thereof is to provide a method for producing a multilayer ceramic substrate excellent in dimensional accuracy and flatness with ease at low cost and capable of eliminating generation of the deformation etc. of the periphery of the cavity.